This invention relates to devices employing interdigitated capacitors for the detection and measurement of the concentration of selected non-aqueous fluids, i.e., gases and liquids, or specific non-aqueous materials or particles, i.e., ions, molecules, or the like in the presence of fluids.
Capacitors of the type having interdigitated plates or fingers have been known and are commonly available in the prior art. These devices have been used in various embodiments to indicate moisture in the atmosphere. For example, R. L. Stevens, et al. in U.S. Pat. No. 2,219,497 describes an electrostatic type test electrode having interdigitated finger-like electrodes covered by a layer of hydroscopic material adapted to absorb moisture from the air in a known relation to the relative humidity of the air. Stevens discloses several capacitor constructions including straight-line interdigitation and concentric circles attached to opposite electrodes placed upon a substrate.
Further, Suntola, in U.S. Pat. No. 4,164,868, describes a capacitive type humidity transducer having a pair of electrically conductive coatings spaced from each other with a dielectric film having water absorption characteristics covering portions of the electrically conductive coatings. Thereafter, an outer electrically conductive, water-permeable layer is carried by the dielectric film covering the electrically conductive coatings which comprise the capacitor plates.
In Suntola, the resultant change in capacitance measured between the plates of the capacitor as modified by the water absorption film is detected by incorporation of the capacitive humidity transducer into a suitable electrical circuit providing measurement of the capacitance and change in capacitance.
Example of suitable electrical circuits to measure capacitance of capacitance type transducers are known in the field, for example, in Review of Scientific Instruments, Volume 44, No. 10, October 1973, authors Dean R. Harrison and John Dimeff illustrate a diode-quad type bridge circuit for use with capacitance transducers wherein a very accurate method of measuring capacitance of an unknown capacitor is provided by placing the unknown capacitor in the diode-quad bridge circuit in series with known capacitors and in parallel to a voltage frequency source. The output voltage of the diode-quad bridge circuit, with a stabilized input voltage frequency source, is a DC voltage which is a function of the difference between the capacitance sought to be measured and the capacitance of the known capacitor.